1. Field of the Invention
This invention relates to semiconductor field effect devices having high speed and high frequency performances, and more particularly to field effect transistors using one or more compound semiconductor materials.
2. Description of the Prior Art
Schottky gate field effect transistors (hereinafter, referred as MESFET's) for high frequency and high speed applications recently use GaAs having an electron mobility which is five to six times greater than Si. In such MESFET's, a GaAs layer is deposited on an insulator substrate and is heavily doped with N-type impurities, for example 10.sup.17 cm.sup.-3. A Schottky metal layer is contacted with the GaAs layer to form a gate electrode. The electron mobility, however, is limited to a low value, for example 5,000 cm.sup.2 /V.Sec at 300.degree. K., due to the scattering of ionized impurities.
To obtain an MESFET free from the effect of the scattering of ionized impurities, T. Mimura has proposed in U.S. Pat. No. 4,424,525 a usage of heterojunction structure in which an AlGaAs layer doped with donor impurities is deposited on a high purity GaAs layer. A Schottky gate electrode is contacted with the AlGaAs layer. With the heterojunction structure, electrons in the impurity doped AlGaAs move into the high purity GaAs to form an electron accumulation layer, that is so-called two-dimensional electron gas, in the high purity GaAs at the interface between the high purity GaAs layer and the impurity doped AlGaAs layer. The condition for forming the two-dimensional electron gas is that the electron affinity of the GaAs is higher than the AlGaAs. By the relationship between the electron affinities, a potential well for forming the two-dimensional electron gas is formed in the GaAs layer. Since the two-dimensional electron gas is produced in the high purity GaAs layer, the movement of electrons is not interfered with by the ionized impurities and results in an improved high mobility such as 6,000 cm.sup.2 /V.Sec at 300.degree. K. or 20,000 cm.sup.3 /V.Sec at 77.degree. K. The mobility is superior at a low temperature.
The proposed heterojunction MESFET achieves the improvement of high frequency and high speed characteristics to a considerable extent. It, however, still has a problem at high field operation. If the electrons in the two-dimensional electron gas are excited so as to exceed the upper energy level of energy sub-bands by the high electric field between source and drain regions, the electron mobility lowers due to inter-sub-bands scattering. Especially, since the energy level difference between energy sub-bands at the two-dimensional electron gas region in the proposed heterojunction MESFET is small, the inter-sub-bands scattering is apt to occur even by a small electrical field between source and drain regions. Thus, the high frequency and high speed operation of the proposed MESFET is easily deteriorated by the operating voltage between source and drain regions.
The proposed heterojunction MESFET has another problem. If the drain voltage V.sub.D is increased to exceed a voltage difference between the gate voltage V.sub.G and the gate threshold voltage V.sub.T, the potential well for forming the two-dimensional electron gas disappears near the drain region and the energy band structure of the channel region near the drain region declines from the interface portion between the AlGaAs and GaAs layers to the inner portion of the GaAs layer. This allows the electron flow to expand into the inner portion of the GaAs layer and results in an increment of drain-conductance in saturation operation. The drain-conductance increment is remarkable in short-channel MESFET's.